The Earth Similarity Index, ESI or “easy scale” is a measure of how physically similar a planetary mass object is to Earth. It is a scale from zero to one, with Earth having a value of one. The ESI was designed to measure planets, however the formula can also be applied to large natural satellites and other objects. The ESI is a function of the planet’s radius, density, escape velocity, and surface temperature. These parameters are often estimated based on one or more known variables. Such variables depend greatly on the method of observation used. For example, surface temperatures is influenced by a variety of factors including irradiance, tidal heating, albedo, insolation and greenhouse warming. Where these are not known, planetary equilibrium temperature is frequently used, or the variable is inferred from other known attributes.